[llvm] [LICM][SimplifyCFG] Ignore frees for writable dereferenceability check (PR #202589)

[Nikita Popov via llvm-commits]

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@@ -2149,9 +2149,13 @@ bool llvm::promoteLoopAccessesToScalars(
   if (StoreSafety == StoreSafetyUnknown) {
     Value *Object = getUnderlyingObject(SomePtr);
     bool ExplicitlyDereferenceableOnly;
+    // The dereferenceability query here is only required to satisfy the
+    // writable contract, actual dereferenceability has already been proven
+    // above. As such, we can ignore frees.
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nikic wrote:

Not quite. Frees in the loop are not really a concern here, because they would prevent promotion entirely (by being an unanalyzable write). What this code is about is determining whether it's legal to introduce a speculative store. Dereferenceabiliy is a necessary but not sufficient condition for this, we also need that it is a non-escaped writable object. If this is ensured by `writable`, then we also need to check this condition from LangRef:

> This attribute is only meaningful in conjunction with dereferenceable(N) or another attribute that implies the first N bytes of the pointer argument are dereferenceable.

That's what this separate dereferenceability check is for.

https://github.com/llvm/llvm-project/pull/202589